An optical filter has a function of transmitting only light in a specific wavelength band and blocking light in other wavelength bands (hereinafter, referred to as wavelength transmission characteristics), and is disposed, for example, on an image sensor (photodetector) which detects light in a diffraction grating spectral device. In this spectral device, light in a certain wavelength band output from a light source is made incident on a sample, and light transmitted through the sample of the incident light is spectroscopically divided by a diffraction grating. Only light in a wavelength band to be detected of the spectroscopically divided light is transmitted through the optical filter, and received and detected by the image sensor. On the other hand, light in a wavelength band other than the detection target, such as high-order diffracted light including second-order diffracted light and third-order diffracted light with wavelengths shorter than that of light of the detection target are blocked by the optical filter and are hardly detected by the image sensor.
As an example of the above-described optical filter, an optical filter is described in FIG. 3 of Patent Document 1 listed below, which is configured by forming bonding surfaces of a plurality of optical filters disposed immediately in front of the image sensor diagonally to the alignment direction of detection elements (sensor pixels) of the image sensor, and bonding adjacent optical filters to each other by an adhesive. The Patent Document 1 describes that, in a spectral device configured by using this optical filter, no step which makes inaccurate the spectral characteristics in acquired spectral data is formed, so that accurate quantitative and qualitative analysis is realized.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-156343